In a traditional power generation process, a fuel (such as coal, natural gas, or syngas) is combusted in the presence of oxygen, producing a stream of hot, high-pressure gas. This hot, high-pressure gas is then used to drive a gas turbine, which in turn drives a generator, producing electrical energy. The exhaust gas from the turbine is still very hot and may contain as much as 50% of the energy generated by the combustion process. This remaining heat (i.e., hot exhaust fumes) is wasted.
In recent years, there has been considerable interest in combined cycle power generation to improve the energy efficiency of the process. A combined cycle power plant generates additional electricity by using the hot exhaust gas from the gas turbine to boil water to make steam. The steam, in turn, is used to drive a secondary steam turbine, generating additional electricity. Combined cycle power generation processes are well-known in the art and are described, for example, by Rolf Kehlhofer et al. in Combined-Cycle Gas & Steam Power Plants (3rd ed., PennWell Corporation; Tulsa, Okla., 2009).
Although the combined cycle power generation process is inherently more expensive than the more traditional, gas turbine-only power generation process due to the additional capital equipment required, it is expected that the additional energy generated will eventually more than off-set the cost of the additional equipment. As a result, most new gas power plants in North America and Europe are combined cycle.
However, regardless of whether a traditional or a combined cycle process is used to generate electrical power, combustion of gaseous fuels produces exhaust gases contaminated with carbon dioxide that contribute to global warming and environmental damage. Such gas streams are difficult to treat in ways that are both technically and economically practical, and there remains a need for better treatment techniques. Treatment is also needed for exhaust gases produced by other fossil fuel-burning furnaces, ovens, and boilers.
Gas separation by means of membranes is a well-established technology. In an industrial setting, a total pressure difference is usually applied between the feed and permeate sides, typically by compressing the feed stream or maintaining the permeate side of the membrane under partial vacuum.
Although permeation by creating a feed to permeate pressure difference is the most common process, it is known in the literature that a driving force for transmembrane permeation may be supplied by passing a sweep gas across the permeate side of the membranes, thereby lowering the partial pressure of a desired permeant on that side to a level below its partial pressure on the feed side. In this case, the total pressure on both sides of the membrane may be the same, the total pressure on the permeate side may be higher than on the feed side, or there may be additional driving force provided by keeping the total feed pressure higher than the total permeate pressure.
Using a sweep gas has most commonly been proposed in connection with air separation to make nitrogen or oxygen-enriched air, or with dehydration. Examples of patents that teach the use of a sweep gas on the permeate side to facilitate air separation include U.S. Pat. Nos. 5,240,471; 5,500,036; and 6,478,852. Examples of patents that teach the use of a sweep gas in a dehydration process include U.S. Pat. Nos. 4,931,070 and 5,641,337.
Configuring the flow path within the membrane module so that the feed gas and sweep stream flow, as far as possible, countercurrent to each other is also known, and taught, for example in U.S. Pat. Nos. 5,681,433 and 5,843,209.
The use of a process including a membrane separation step operated in sweep mode for treating flue gas to remove carbon dioxide is taught in co-owned, allowed U.S. patent application Ser. No. 12/734,941 (hereinafter referred to as “the '941 application”), filed Jun. 2, 2010, and in co-pending U.S. patent application Ser. Nos. 12/807,846, filed Sep. 13, 2010; 13/071,331, filed Mar. 24, 2011; 13/122,136, filed Mar. 31, 2011; 13/123,342 and 13/123,364, both filed Apr. 8, 2011.